In the case of unexpected grid errors, the wind turbine can no longer feed the maximum power into the electrical grid. The quick drop in power resulting therefrom leads to a rotational speed increase which is correspondingly reacted to by a control of the blade pitch angle. As a result, fluctuations in the rotational speed can occur.
U.S. Pat. No. 7,966,103 discloses a method for operating a wind turbine at a low grid voltage. In the event of a grid error when there is low grid voltage present, the pitch angle at the current power is determined based on the tip speed ratio prior to the occurrence of the grid error. A control for the blade pitch angle is triggered in order to hold the electricity generated constant during the grid error.
U.S. Pat. No. 6,921,985 B2 discloses a method for operating a wind turbine in the event of a grid error in which an uninterrupted power supply for the blade pitch angle control is provided. The uninterrupted power supply here falls back on the intermediate circuit between the grid-side converter and the rotor-side converter. With the switch to the uninterrupted power supply, the blade pitch angle control is activated by the control of the wind turbine in order to correspondingly adjust the blade angle in reaction to the switch process.
U.S. Pat. No. 7,834,472 discloses a method for controlling a wind turbine in the event of a grid error. In the event of a grid error, an operating parameter of the wind turbine, for example the temperature, is captured and the blade angle control is triggered in order to keep the temperature within a predetermined interval.
U.S. Pat. No. 7,851,934 discloses a method for controlling a wind turbine in the event of a grid error, in which method, during a grid error, the blade angle is increased until the rotor no longer accelerates. The control of the rotational speed which is typical during normal operation is not performed during a grid error. The increase in the blade angle is triggered when the active power output by the wind turbine is 125% of the nominal active power.
U.S. Pat. No. 7,714,458 discloses a method for operating a wind turbine in the event of a grid error, in which method a setting of the blade pitch angle is already initiated before the increase in the rotor rotational speed, the rotor rotational speed or generator rotational speed being adjusted to a set-point rotational speed which results from an electrical variable and the current wind speed present in the event of a grid error.